This invention relates to a method of delivering fuel and air to a fuel cell system, and more particularly to a method of removing sulfur-containing species from a liquid hydrocarbon fuel used to produce a hydrogen source for the fuel cell.
In proton exchange membrane (PEM) fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst fouling constituents, such as carbon monoxide (CO) and sulfur.
For vehicular applications, it is desirable to use a liquid fuel such as gasoline as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard, and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam (and sometimes air) to yield a reformate gas comprising primarily hydrogen and carbon dioxide. In an autothermal gasoline reformation process, steam, air and gasoline are reacted in a primary reactor that performs two reactions. One is a partial oxidation reaction (POX) and the other is steam reforming (SR). The primary reactor produces a reformate stream comprising primarily hydrogen, carbon dioxide, carbon monoxide, nitrogen and water. Downstream reactors may include water/gas shift (WGS) reactors for reacting carbon monoxide with water to create hydrogen and carbon dioxide and preferential oxidation (PROX) reactors for selectively oxidizing carbon monoxide in the presence of hydrogen to produce carbon dioxide (CO2), using oxygen from air as an oxidant.
One of the greatest challenges to the development of a fuel cell-powered vehicle using on-board fuel processing of liquid hydrocarbon fuel, such as gasoline, for H2 generation is the ability to remove sulfur from the gasoline. Gasoline produced in the United States today contains at least 30 ppm sulfur by weight. The sulfur is present in a variety of organic sulfur-containing species such as mercaptans, sulfides, disulfides, tetrahydrothiophenes, thiophenes, and benzothiophenes. Typical fuel processor systems contain catalysts to perform reforming, water gas shift, and preferential oxidation reactions. These catalysts, particularly the water gas shift catalysts, are irreversibly poisoned by sulfur. Furthermore, the catalysts at the electrodes of a fuel cell stack typically contain platinum, which is irreversibly poisoned by sulfur. Therefore, it is crucial that the sulfur level in gasoline be reduced to levels tolerable by the fuel cell system catalysts.
One way to remove the sulfur is to place a sulfur trap downstream of the primary reactor (which may be a steam reformer, a partial oxidation reactor, or an autothermal reactor). The primary reactor converts the hydrocarbon fuel to a reformate stream which comprises primarily H2 and CO, and converts essentially all of the sulfur in the fuel to hydrogen sulfide (H2S). A material such as zinc oxide (ZnO) may then be used to adsorb the H2S. However, the ZnO must be at least 300xc2x0 C. to effectively adsorb the H2S. Upon starting up the fuel cell system from low temperatures, the ZnO would be too cold to adsorb the H2S. The H2S would then be allowed to flow to reactors downstream of the sulfur trap, such as the water gas shift reactor and the preferential oxidation reactor (PrOx), and poison the catalysts in those reactors. Additionally, ZnO is not effective at removing H2S to low levels in the presence of water, and gasoline fuel processor systems may contain upwards of 30% water in the effluent from the primary reactor. Furthermore, a ZnO-containing sulfur trap offers no protection for the catalysts in the primary reactor, whose performance may be adversely affected by the presence of sulfur.
Another option is to remove the sulfur directly from the fuel before it enters the primary reactor. It would be desirable to be able to adsorb the sulfur-containing species at room temperature. However, while there has been significant research in that area, to date no materials have been identified which will adsorb all of the types of sulfur species that are present in gasoline in the presence of a liquid hydrocarbon mixture which contains olefins and aromatics. The most promising technology uses a nickel (Ni) catalyst to react with the organic sulfur-containing species to form nickel sulfide (NiS) and desulfurized hydrocarbons as disclosed in U.S. Pat. No. 6,159,256. The reaction can occur at temperatures ranging from 200xc2x0 F. to 525xc2x0 F. A portion of the desulfurized liquid fuel can be collected and kept in a separate fuel tank. The desulfurized fuel can then be used to start up the fuel processor at low temperatures, before the Ni-containing sulfur trap reaches its operating temperature. However, some heat still needs to be provided to the system at startup to vaporize the sulfur-free liquid fuel.
U.S. Pat. No. 5,292,428 discloses a method for desulfurization of liquid hydrocarbon fuel utilizing hydrodesulfurization. Hydrodesulfurization involves reacting the fuel with H2 in the presence of a catalyst to convert the organic sulfur-containing species to H2S, and subsequently adsorbing the H2S on ZnO. This process is not suitable for automotive applications because it requires two unit operations (hydrotreating reactor and ZnO H2S scrubber) that consume valuable space in a fuel cell system. U.S. Pat. No. 6,042,798 discloses a process in which sulfur-containing organic species are removed from a hydrocarbon mixture in a single unit by passing said hydrocarbon mixture over a co-precipitated copper-zinc catalyst in the presence of hydrogen. U.S. Pat. No. 6,184,176 discloses a similar process that uses a sorbent comprising a mixture of zinc oxide, silica, alumina, and reduced cobalt to desulfurize a hydrocarbon stream in the presence of hydrogen. Such processes are not suitable for automotive applications because they operate at high pressures and temperatures and utilize hydrogen, which requires a recycle of the H2 product from the fuel processor.
The current invention provides a method for supplying air and sulfur-free hydrocarbon fuel vapor upon start-up of a fuel cell engine, as well as a means for replacing the supply of sulfur-free hydrocarbon fuel vapor during normal engine operation. No external heat is required to provide the air/fuel vapor mixture to the inlet of the fuel processor system.
This invention includes a method for removing sulfur-containing species from a liquid hydrocarbon fuel and capturing a portion of vaporized sulfur-free fuel to be processed into hydrogen for use in a fuel cell engine. Sulfur is removed from a hydrocarbon fuel such as gasoline, diesel, or kerosene by heating the fuel under pressure to keep the fuel in the liquid phase, and passing it over a sulfur trap that contains an adsorbent bed that adsorbs the sulfur-containing species in the fuel. The sulfur-free fuel that exits the adsorbent bed is slightly depressurized to generate a two-phase hydrocarbon mixture. The vapor/liquid mixture is separated, and the liquid portion is sent to the inlet of a fuel processor system where it is mixed with air and steam to produce a hydrogen-rich reformate mixture. The vapor portion of the sulfur-free hydrocarbon mixture is sent to a vapor canister where it is adsorbed on an activated carbon adsorbent. The adsorbed hydrocarbon vapors are desorbed from the vapor trap by purging it with air when the fuel cell engine is first started up. The mixture of air and sulfur-free hydrocarbon fuel is sent to a thermal start device or to the fuel processing system to be used to rapidly start the fuel cell engine using sulfur-free fuel before the liquid-phase sulfur trap has reached an operating temperature sufficient to remove sulfur components from the liquid fuel. This method therefore provides a means to remove sulfur from a liquid hydrocarbon fuel to be processed into hydrogen and a means to rapidly start the fuel processor system by utilizing captured sulfur-free hydrocarbon vapor.
These and other objects, features and advantages of the present invention will be apparent from the following brief description of the drawing, detailed description of the preferred embodiments and appended claims and drawing.